Even with a prime lens, I seem to be able to do noticeably better with my 450D by manually focusing using live view at maximum zoom than by using auto focus (even with just the center focus point selected and aimed at the same high contrast region that I am looking at in live view). However this is a painstaking procedure that requires a tripod and a fair bit of time; not really suitable for capturing fleeting expressions on people's faces or rapidly moving children.

Unlike my old film camera, the 450D lacks a prismatic focusing screen, and my eyesight isn't good enough to do better than the autofocus through the viewfinder without one. How do others deal with focusing for candid portraits? Any tips?

What sort of pictures are we talking? Candids of people at an event, sports? Animals? With your particular details, it seems like we're missing an important part of the equation to know what tips to give.
– jfklein13Aug 4 '10 at 22:09

7 Answers
7

The lack of focus could be partially the result of camera shake. Even though the focus is correct, the image may be blurred slightly by the movement, which would go away using a tripod.

If you are looking at 100% crops of digital images, there will probably be some blur unless you make sure to select a high enough shutter speed. The same image viewed as a 4x6 is going to be quite sharp compared to the 100% crop.

+1. OP, you need to do these tests such that everything is consistent between the two conditions except what you are testing. And don't pixel peep. It's unnecessary and will make you sad.
– ReidAug 4 '10 at 23:30

+1 Definitely on the obscure side of things, but well worth a mention!
– ex-msAug 5 '10 at 0:32

I never even knew about it until now. I've always hated how the 450D focused, but its extremely difficult to manually focus given its tiny viewfinder. I am hoping a focus screen will solve the problem.
– jrista♦Aug 5 '10 at 0:49

my experience (with a creaky old D100) is that they're an improvement, but not a miracle. Viewfinder's still tiny, and only slightly less dim, but manual focus is improved. Mind you, I was using it with MF-only lenses.
– ex-msAug 5 '10 at 1:28

I generally manual-focus my 100mm macro lens, but I have not been able to get any macro floral or insect shots that, once viewed on my computer, were worth keeping due to poor focus. Even if it helps only a little, I think it would still be worth it. I figure I'll start with the $20 one and try it, and if it is useful, maybe go for the $200 one.
– jrista♦Aug 5 '10 at 3:28

There have been claims of back and front focus issues on the Canon 450D, but I'm not entirely sure I believe them. I have a friend with that camera that runs around with an unstabilized 400mm Canon prime and takes sharp bird pictures using that lens. So... I suspect that it may be more related to camera shake caused by one or more of the following:

Mirror vibration which can have bigger impact on slow shutter speeds.

Technique which can cause camera shake. For example, if you stab at the shutter release, that can cause movement.

Slow shutter. Hand holding a shot with a shutter speed well below 1/f (f being focal length) is sure to introduce shake, especially with an unstabilized lens.

Low light focus hunting. You may have shifted focus as you pressed the release and not realized it.

After all that, it is entirely possible that you have a front or back focussing issue but, as I said, I suspect the general claims of this are unlikely.

Every component (lens and body) have independent tolerances when it comes to focusing. With a the shallower the depth of field, the overall focus offset becomes apparent.

To make things more complicated, the focus offset may change with these variables: focus distance, aperture, and focal length.

Although it is entirely possible that your combination may be front or back focusing, you should look at other factors such as camera shake.

If you want to determine if it really is front or back focusing, or something else, do a focus test. It's important you do this outside with natural light, as artificial, particularly incandescent, will cause front focus with most cameras. If it is calibrated indoors, you'll get back focus with other light.

Another thing to think about: If you do change the focusing screen, manually focusing will get quicker over time. You'll learn your lens and you'll be able to do it much faster. Practice, practice, practice!